The present invention relates to optical testing equipment and in particular to equipment for testing far field optical equipment such as laser communication transceivers, missile trackers, target acquisition systems and telescopes.
Optical systems used for long distance acquisition, tracking, observation, or communication are widespread in both government and commercial applications. These systems include military target acquisition systems, missile trackers, telescopes, surveyors, generalized optical communication terminals, and others. These systems may be comprised of one aperture and a corresponding optical system, or multiple apertures and/or multiple optical systems, each working in transmit (Tx) or receive (Rx) or both transmit and receive. If a system has multiple apertures, it is generally necessary that all apertures be xe2x80x9cboresightedxe2x80x9d with respect to each other, meaning that they are optically aligned to point in precisely the same direction. The distances over which these systems operate are typically long enough that they can be optically characterized as having their targets located at infinity, a regime commonly referred to as the xe2x80x9cfar fieldxe2x80x9d. An example of such a system is an astronomical telescope, which images all stellar objects as though they were identically located at infinity, despite the fact that the actual distances may vary by orders of magnitude. In the far field these optical devices typically do not distinguish distance, but instead view all target objects as though they are at infinity. Prior to deployment it is sometimes impractical to test optical devices at the distances over which they are designed to operate. An alternative is to use a device known as a xe2x80x9cfar field simulatorxe2x80x9d (FFS). A FFS is an instrument that is placed in the proximity of the optical device under test and aligned such that the two systems are staring directly at one another. The FFS objective aperture should be large enough to encompass any and all of the apertures and subapertures of the optical device under test. The FFS then simulates the optical characteristics of targets in the far field, allowing performance measurements of all pertinent parameters of the device under test.
Often it is desirable to utilize the FFS under conditions characteristic of the field of deployment as well as those found in controlled laboratory conditions. Because optical systems may be deployed under extreme and diverse conditions, the FFS must be capable of operating over a large range of ambient temperatures, humidity, and other local conditions. As an example, an aircraft laser communication terminal may have to be performance verified under a tarmac test temperature range of from xe2x88x9220 to +120 degrees Fahrenheit. These extremities raise inherent difficulties in designing the FFS to maintain reliable calibration precision under all test circumstances. Designing a FFS that maintains calibration in field conditions as well as in the laboratory is not always practicable.
What is needed is an FFS that is capable of precise simulation under all applicable situations and with the capability of quick calibration and easy confirmation that the system is calibrated accurately.
The present invention provides a far field simulator with precision that can be directly quantified under any applicable test conditions. The FFS is easily adjusted such that rays emanating from a point focus traverse the FFS, reflect back upon themselves precisely retracing their paths, and are perfectly self-imaged back to the original point. Optical interference at the self-imaged point can be used to verify proper calibration of the FFS with great accuracy. The FFS may be used in the laboratory or in the field of operations for testing of optical equipment used at great distances. The nature of the system is such that the required optical elements remain relatively simple. Alignment tolerances for an FFS are extremely strict in order to guarantee that any optical errors measured are in the equipment under test and not in the FFS test apparatus. These alignment tolerances must be maintained in both inclement field and controlled laboratory environments. Preferred embodiments provide this precision without requiring a metering structure of sufficient stiffness and thermal compensation to guarantee alignment over a range of temperatures and other environment conditions common to these varied environments. These preferred embodiments include an FFS with an autostigmatic cube an objective mirror and interferometer optics for calibrating the FFS with precision. The autostigmatic cube provides an image plane and includes a beam splitter, a pinhole and an optic defining a reference sphere having, by reason of the beam splitter, two optical centers of curvature. A first optical center of curvature is located at, or approximately at the image plane and a second optical center of curvature is located at the pinhole. The objective mirror has (also by reason of the beam splitter) a first focus and a second focus. It is positionable in tilt, tip and piston such that the first focus is located at the first optical center of curvature (i.e., at the image plane) of the autostigmatic cube and the second focus is located at the cube""s second center of curvature (i. e., the pinhole). An imaging microscope objective is provided which magnifies the image plane and relays it to a view plane where a CCD is located. The FFS is calibrated by adjusting the objective mirror to null out interference fringes at the image and view planes. The approach of the present invention provides a means to verify the FFS to a sub-wavelength level of precision so that it can be adjusted and calibrated at the time of test in any applicable test environment. This provides the user access to full knowledge of the FFS""s optical characteristics under any conditions, such that they can be removed as variables in the test, allowing the tester to isolate the performance characteristics of the equipment under test.